This invention relates to a process for the separation of components of gaseous mixtures, and particularly relates to an improved selective adsorption process for the separation of high purity light gases, especially hydrogen or helium.
Cyclic swing adsorption or pressure swing adsorption referred to herein as selective adsorption has been widely used for the recovery of lighter, less sorbed gases from a mixture with one or more heavier, more readily adsorbed gases.
The light gas desired in relatively more purified form is frequently hydrogen. Such hydrogen may be recovered from various hydrogen-containing gas mixtures such as purge streams from various synthesis processes involving hydrogen as a reactant, such as hydrogenations, syntheses of ammonia or hydrocarbons, or as a product or by-product, such as dehydrogenation reactions as well as gas mixtures produced by the controlled combustion or reforming of hydrocarbons or from cracking of hydrocarbon feedstock. Another light gas which also is recovered in more purified form by the use of selective adsorption is helium which can be recovered from helium-containing natural gas compositions.
Selective adsorption systems generally involve passage of the feed gas mixture through equipment comprising two or more adsorbers containing beds of molecular sieves or other adsorbents for the heavier components of the gas mixture. The selective adsorbers are arranged to operate in sequence with suitable lines, valves, timers and the like so there are established an adsorption period during which the heavier components of the feed gas mixture are adsorbed on the molecular sieve or other adsorbent and a regeneration period during which the heavier components are desorbed and purged from the adsorbent to regenerate it for reuse. Those selective adsorption processes which operate by cycling from low temperatures to high temperatures utilize a period of cooling gas flow to restore adsorption conditions. The more frequently employed pressure cycled processes utilize a repressuring period during which the adsorber is brought substantially to adsorption period pressure.
There are frequently added one or more guard adsorbers upstream from the primary selective adsorbers containing an adsorbent or adsorbents, which may differ from the primary selective adsorbent, used to adsorb any unusually heavy contaminants in a feed gas mixture which might tend to inactivate the primary adsorbent or render it difficult to regenerate, including such as vapors or normally liquid hydrocarbons and the like. The regeneration or purging of the primary selective adsorbers and the guard adsorbers in earlier adsorption processes was generally carried out by recycling at a lower pressure or at a higher temperature a part of the product purified light gas. Such processes of selective adsorption have been widely described in the literature and are familiar to those skilled in the art.
Although these selective adsorption processes are effective to product a relatively highly purified light gas product, they suffer from disadvantages. The greatest disadvantage is the relatively low percentage recoveries which are generally encountered when a substantial part of the light gas product is used for the regeneration of the primary adsorbers and/or guard adsorbers. Recoveries of hydrogen gas generally range from about 50 to about 80%. A further disadvantage is the fact that the substantial portion of the light gas product used for regeneration of the primary adsorbers and guard adsorbers cannot be economically recovered from the regenerating gas mixtures and such mixtures including the components separated by the guard adsorbers and primary adsorbers are generally disposed of by venting, flaring or employing as low grade fuels.
Subsequently improved selective adsorption processes have been suggested wherein the regeneration of the primary selective adsorbers and/or guard adsorbers employ a portion of the substantially purified void space light gas from a selective adsorber whose adsorptive capacity has not been fully occupied by adsorbed heavier components but a portion of which adsorbent pores or "void space" is occupied by purified light gas, particularly that portion of the adsorbent at the downstream end of the adsorber. Such substantially purified light gas for regeneration can be stored in a separate vessel as taught by U.S. Pat. No. 3,142,547 of Marsh et al, or it can be used directly in a system of four or more sequenced adsorber beds as taught by U.S. Pat. No. 3,430,418 of Wagner. Although such improved selective adsorption processes have reduced the amount of purified light gas product utilized for regeneration and repressuring and have increased the recoveries of the desired light gas product, the light gas disposed of along with the desorbed and purged components represents loss of desired product. Thus, further improved recoveries of purified light gas product are desired.
It has also been suggested to employ a low temperature separator unit in conjunction with a pressure swing adsorption unit with recompression of the regenerating gas mixture from the adsorbers and recycle to the low temperature separator unit as taught by U.S. Pat. No. 3,838,553 of Doherty. Such systems are complicated and costly, requiring an attendant refrigeration system for operation of the low temperature separation unit. Hence, such systems have not been widely used.
There has now been developed a selective adsorption process which produces both high purity light gas and a high yield of such gas from the gas mixture or mixtures containing the light gas. The process involves the use of a recovery system comprising a light gas selective permeator and a selective adsorption unit with recycle of a substantial portion of the light gas separated by the permeator to the selective adsorption unit for recovery or reuse.